Microstructural Effects on the Fatigue Properties of a Cast Al7SiMg Alloy 840121
The fatigue strength of permanent mould and sandcast aluminium is strongly influenced by the cooling rate during casting. Slower solidification yields larger secondary dendrite arm spacing with a coarser eutectic and greater defetcs i.e. porosities and intermetallic β (FeSiAl5).
The negativ effect of porosity is greater than that of the amount/size of the β-platelets. The defects affect both the crack nucleation and the propagation phases. For very large defects, the initiation phase is reduced and a large portion of the life is spent as crack propagation.
The secondary dendrite arm spacing (DAS) represents a good over all indication of the fatigue strength of the material, which is proportional to (DAS)−1/2. This relationship can be used to estimate the fatigue strength at various locations in a cast aluminium component.